Ultrasonic diagnosis device

ABSTRACT

This invention provides a technology that freely changes the frequency of a reference signal in a digital ultrasonic diagnosis device and makes it possible to conduct various high-quality or high-speed imaging. Reception signals obtained by an ultrasonic element group disposed in an array are digitized in a sampling cycle higher than a Nyquist frequency of the reception signals, and are mixed with reference signal. The mixed signals are then accumulated in a sampling direction, a number of the mixed signals accumulated being equal to a number of a plurality of samples, and are delayed and added by wave reception focusing means. The reference signal serially read out and used from memory means in such a manner as to correspond to the sampling number of the reception signals. A reference signal train acquired from signals, the frequency of which decreases gradually with a lapse of time, from wave transmission, or a reference signal train obtained alternately from two signals having different frequencies, may be stored in advance in memory means.

This is a divisional application of U.S. Ser. No. 09/555,044, filed May24, 2000 now U.S. Pat. No. 6,373,140, which is a 371 of PCT/JP98/05340,filed Nov. 27, 1998.

TECHNICAL FIELD

This invention relates to an ultrasonic diagnosis device. Moreparticularly, this invention relates to a technology that will be usefulfor digitization of a signal processing that acquires tomograms havinghigh image quality irrespective of an object and an imaging portion, inan apparatus for non-destructively inspecting an object by ultrasonicwaves or in an ultrasonic diagnosis device for conducting medicaldiagnosis.

BACKGROUND ART

A method called a “frequency shift beamforming method” is known as oneof the signal processing methods for forming ultrasonic tomograms byusing reception signals from ultrasonic transducer elements that aredisposed in an array. This method comprises the steps of mixing thesignal from each ultrasonic element with a reference signal, extractinga low frequency component of each mixed signal, or in other words,moving it towards the low frequency side, delaying the mixed signals byrespective delay circuits to beamforming, then adding the signals, andobtaining ultrasonic response signals focused to desired positions.References associated with the present invention include U.S. Pat. No.4,140,022, JP-A-52-20857 and U.S. Pat. No. 4,983,970.

JP-A-6-313764 discloses the technology that is based on the frequencyshift beamforming method described above and the major proportion of thesignal processing circuit is a digital circuit. This technology employsparticularly an over-sampling technique that executes an accumulationprocessing after each sampled ultrasonic signal is mixed with areference signal, and improves effective accuracy of analog-to-digitalconversion.

On the other hand, JP-A-9-206298 discloses an ultrasonic diagnosisdevice employing digital orthogonal wave detection. In this reference, asignal from each ultrasonic transducer element is delayed by a delaycircuit. Next, the delayed signal is inputted to an orthogonal detectioncircuit, and the in-phase component and the orthogonal component aregenerated. This orthogonal detection circuit executes interpolation thatis equivalent to changing the frequency of the reference wave to copewith a spectrum shift in which the center frequency of an ultrasonicecho shifts to a low frequency range with the echo time.

DISCLOSURE OF INVENTION

It is not easy to drastically change the frequency of the reference wavein a system which executes the effective change of the reference wave byinterpolation. In the digital orthogonal detection system of the knownreference described above, the high frequency signal generated bydigitizing the ultrasonic signal from each element is beamformed by thedelaying means before the frequency shifts to the low frequency range.Therefore, high accuracy is necessary for controlling the delay time ofeach signal channel.

The present invention is based on the premise of the frequency shiftbeamforming method that exploits the effect of over-sampling. In otherwords, an ultrasonic signal from each element is sampled at a frequencyhigher than a Nyquist frequency of the upper limit of its signal band,and is subjected to an accumulation processing in a subsequent stage.The digital signal so sampled is then mixed with a reference signal andis beamformed by a delaying circuit after the accumulation processing isdone and then added.

In such a digitized ultrasonic diagnosis device in accordance with thefrequency shift beamforming method, it is one of the objects of thepresent invention to freely vary the frequency of the reference signaland to make it possible to conduct various high-quality or high-speedimaging.

In an ultrasonic diagnosis device comprising primarily digital circuits,it is one of the more concrete objects of the present invention toprovide a technology capable of conducting harmonic imaging.

In an ultrasonic diagnosis device comprising primarily digital circuits,it is another object of the present invention to provide a technologycapable of simultaneously forming a plurality of beams having differentfrequencies.

It is a further object of the present invention to improve resolution ina lateral direction and to thus improve quality of ultrasonic images.

The above and other objects and novel features of the present inventionwill become more apparent from the description of the specification inconnection with the accompanying drawings.

A typical construction of an ultrasonic diagnosis device according tothe present invention comprises wave transmitting means for repeatedlydriving an ultrasonic element group disposed in an array and generatingultrasonic waves; digital converting means for sampling receptionsignals obtained by the ultrasonic elements at a sampling frequencyhigher than a Nyquist frequency of the upper limit of the signal bandand converting them to digital signals, respectively; mixing means formultiplying the digital signals by a reference signal, respectively;accumulating means for accumulating the multiplied digital signals forplurality of samples, respectively; receive focusing means for impartinga delay to each digital signal so accumulated for aligning the phasedifference peculiar to each ultrasonic element and adding the digitalsignals; and mixing data generating means for supplying serially thereference signal to the mixing means. More concretely, the mixing datagenerating means includes data computing means for computing in advancea train of reference signals corresponding to the sample point number ofthe reception signals, and memory means for storing the reference signalline so computed and outputting serially the stored reference signalsunder read address control corresponding to the sample number of thereception signals.

In general ultrasonic tomograms imaging, the mixing data generatingmeans supplies the signal, the frequency of which decreases graduallyfrom near the center frequency of the transmission wave with the lapseof time, as the reference signal train is provided to the mixing means.In consequence, a mixing processing can be conducted in such a manner asto correspond to a spectrum shift in which the frequency band of thereception ultrasonic waves shifts to the low frequency side as an echodepth increases, and quality of the ultrasonic image can be improved.

Data acquired by digitizing signals having a frequency that is somemultiples of the center frequency of the transmission ultrasonic wavesare prepared as a reference signal train and are used serially formixing. In this way, harmonic imaging can be executed easily. In thiscase, too, image quality can be improved by applying means for graduallydecreasing the frequency.

A peculiar construction utilizing the characteristic in which the signalfrom each ultrasonic element is over-sampled can be employed. Namely,wave transmitting means are provided two times to the constructiondescribed above, and two beams having mutually different centerfrequencies ω1 and ω2 and different focus directions are synthesized andtransmitted simultaneously. On the other hand, the mixing datagenerating means generates alternately a reference signal generated froma signal having a center frequency near ω1 and a reference signal from asignal near ω2, and multiplication is effected by using the referencesignals that change alternately. The accumulating means and the receivefocusing means are disposed two times, too, so that the outputs of themixing means can be assorted and supplied by the multiplexer. One of thetwo receive focusing means conducts beamforming in the direction of thebeam of ω1 and the other, in the direction of the beam of ω2. Accordingto this construction, a plurality of ultrasonic beams are synthesized inwave transmission and are transmitted. In the reception signalprocessing, on the other hand, the beams for applying an appropriatebeamforming processing are substantially selected by alternate switchingof the reference signals. In this way, high-speed imaging becomespossible.

It is possible to constitute another construction by utilizing the wavetransmitting means for synthesizing and transmitting a plurality ofultrasonic beams. The wave transmitting means synthesizes an ultrasonicbeam having a center frequency ω1 and a first focus in a near distanceand an ultrasonic beam having a center frequency ω2 which is lower thanω1 and a second focus in the same direction as the first focus but in afar distance, and transmits the synthesized signal. The portion of thereception signal processing is exactly the same as ordinary imagingdescribed above. Mixing is executed by the reference signal trainacquired from the signal the frequency of which decreases gradually.According to this construction, multi transmit focusing is made bysimultaneous wave transmission. Consequently, imaging can be made at ahigh speed while resolution in the lateral direction is improved in abroad depth range.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is an explanatory block diagram showing a schematic constructionof an ultrasonic diagnosis device according to Embodiment 1 of thepresent invention.

FIG. 2 is a block diagram showing a detailed construction of Embodiment1 described above.

FIG. 3 is a waveform diagram of a signal the frequency of which dropswith the lapse of time as the basis of mixing data in Embodiment 1described above.

FIG. 4 is an explanatory block diagram showing a schematic constructionof an ultrasonic diagnosis device according to Embodiment 2 of thepresent invention.

FIG. 5 is a conceptual view showing simultaneous transmission of wavesin two directions in Embodiment 2 described above.

FIG. 6 is a conceptual view showing examples of digitized referencesignals in Embodiment 2 described above.

FIG. 7 is a conceptual view showing a processing of a sample train andan assorting operation.

FIG. 8 is a conceptual view showing simultaneous transmission of aplurality of beams having different focal lengths in Embodiment 2described above.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, embodiments of the present invention will be explained indetail with reference to the drawings. The same reference numeral willbe assigned to a member having the same function throughout thedrawings.

(Embodiment 1)

FIG. 1 is a diagram for explaining a schematic construction of anultrasonic diagnosis device according to Embodiment 1 of the presentinvention. An ultrasonic probe 101 includes N ultrasonic elements thatare arrayed one dimensionally. Each ultrasonic element is repeatedlydriven by a wave transmission signal obtained by imparting a delaydistribution to a driving signal generated by a driving source 85 by awave transmission focusing circuit. Digital conversion means 102comprises N A/D converters. A reception signal of an ultrasonic echo ofeach element is sampled by a sampling clock of a cycle T that is commonto all channels, and is digitized. The sampling cycle T is set to avalue smaller than a Nyquist cycle of the upper limit of the band of areception wave signal. Here, the term “channel” represents one signalcomponent in the process in which N reception signals are subjected tothe same processing in parallel with one another. Each of the digitizedsignals is multiplied by a digitized reference signal in mixing means103. The reference signal to be multiplied will be explained later indetail. However, since it is a discrete value of complex components Rand I of a signal having a frequency ωm substantially equal to a centerfrequency ωS of the reception signal, the multiplication results provecomplex signals.

Furthermore, its frequency band shifts to the band having its center ata subtracted frequency band (base band) between the band of the originalreception signal and ωS, and at a summed frequency (ωS+ωm). Eachmultiplication result is accumulated by accumulating means 104, for aplurality of samples, that is, a number of multiplication resultsaccumulated being equal to a number of a plurality of samples, in thedirection of the lapse of time. The result is only the base band portiondue to a low-pass filter effect brought forth by accumulation.Incidentally, the accumulating means 104 may be not only means forexecuting mere accumulation but also means for executing accumulationwith multiplication by a weighting function, that is, multiplication andaddition computation, to realize desired low-pass filtercharacteristics.

Digital delaying means 105 applies a delay that compensates thedifference of an echo arrival time of each element, to each accumulationsignal. The delay results for the N channels are added by adding means106 to accomplish a well-known electronic focusing effect that amplifiesonly a reflection echo from a target focal point. The output of theadding means 106 can be obtained for each of the real part (in-phasecomponent) and the imaginary part (orthogonal portion) of the complexsignals. Therefore, envelop changing means 92 changes the values toabsolute values and writes the results into a digital scan converter 94.

The operation described above is repeatedly executed in accordance withan electronic sector scan method that is well known in the art. Thedirection of the focal point of the transmission ultrasonic waveaccomplished by a wave transmission focusing circuit 86 is changedwhenever the wave transmission is repeated. The direction of the focalpoint of the reception wave beam, that is accomplished by the digitaldelaying means 105, too, is controlled in match with that of thetransmitting ultrasonic wave. The result of this beam scan is memorizedserially in the digital scan converter 94, and an ultrasonic tomogram isdisplayed by a display 96.

Incidentally, the focal point of the reception beam is moved in adepth-wise direction as the echo depth of the reception signal becomesdeeper with the lapse of time during the wave reception periodsubsequent to one wave transmission operation. This technique, too, iswell known in the art as dynamic receive focusing. The control signal dgiven from a control circuit 90 to the digital delaying means 105 inFIG. 1 represents the signal for controlling the change of the delaydistribution resulting from the focus movement. On the other hand, thecontrol signal b to be given to the wave transmission focusing circuit86 represents the signal for controlling the change of the direction ofthe focal point of the transmission ultrasonic wave described above.

In the ultrasonic diagnosis device described above, the digitizedreference signal multiplied by the mixing means 103 for moving thefrequency band is not the one that is obtained by digitizing a signalhaving a predetermined frequency. In other words, the high frequencycomponent of the reception signal attenuates vigorously with the lapseof time following the transmission of the wave, or in other words, asthe depth of the reflecting sound source becomes greater, the centerfrequency ωS of the reception signal shifts to a lower frequencydirection. Therefore, if the reference signal of a predeterminedfrequency is used, detection efficiency of the ultrasonic echoesdegrades in the deep region. The reference signal supplied from mixingdata generating means 107 to the mixing means 103, too, changes thefrequency in such a manner as to correspond to such a spectrum shift.

The mixing data generating means 107 comprises mixing data computingmeans 109 and mixing data storing means 110 as shown in FIG. 2. Themixing data computing means 109 is materialized by a program running ona known data processing unit. In this embodiment, the sampling cycle Tof the digital converting means is 40 nsec and the number of samples inthe wave reception period after one wave transmitting operation is4,096. Consequently, the mixing data computing means 109 computes thecomplex components of the reference signal expressed by the followingequation for an integer k satisfying the relation 0≦k<4,096:

h(kT)=exp(jωm(kT)kT)  (1)

Incidentally, the chase item should be expressed correctly byintegration, but is expressed in brief by the product of the frequencyand the time in equation (1). FIG. 3 shows the function expressed byequation (1). The frequency ωm(kT) drops monotonously with the lapse oftime from near the center frequency of the transmission wave, that is,with an increasing k value. The degree of this drop may follow the shiftof the center frequency of the reception signal of the ultrasonic wave.When the center frequency of the transmission signal is 3.5 MHz, forexample, the following equation can be employed as ωm(kT) in equation(1): $\begin{matrix}{{\omega_{m}({kT})} = {2{\pi \cdot 3500000 \cdot \left( {1 - {\frac{k}{4095} \cdot \frac{2}{7}}} \right)}}} & (2)\end{matrix}$

In equation (2), ωm(kT) is 3.5 MHz when k=0. Thereafter the frequencydecreases linearly and reaches ωm(kT)=2.5 MHz at k=4095.

The mixing data computing means 109 computes in advance the complexcomponents of the discrete value of the signal, the frequency of whichis variable, in every 40 nsec for all k values of 0≦k<4,096 as adigitized reference signal train, and the mixing data storing means 110stores the computation results. The mixing data storing means 110includes a known semiconductor memory. The address control signalrepresented by c in FIG. 1 serially designates the read address of themixing data storing means 110. The digitized reference signals suppliedare serially read out in such a manner as to correspond to the samplenumber k of the digitized reception signals and are supplied to themixing means. Incidentally, this digitized reference signal is common toeach channel. Generally, the spectrum shift of the reception signalfrequency is not much affected by the direction of the ultrasonic beam.Therefore, the same changing method not depending on the beam directionmay be employed for the frequency change of the reference signal. Inother words, the data read control by sequential designation of theaddress is repeated in exactly the same way whenever the transmissionwave iterates.

When the reference signal of the frequency following the spectrum shiftof the reception signal is employed for mixing as described above, thecenter frequency of the base band component generated by mixing alwaysexists near zero. Consequently, base band extraction by accumulation canbe executed highly efficiently, hence, quality of the ultrasonictomogram can be improved.

In the embodiment given above, the frequency of the reference signalcorresponding to the reception signal from the smallest depth (k=0) isset to be equal to the center frequency 3.5 MHz of the transmissionwave. It may be possible, however, to set the center frequency of thereception wave to be imaged to a higher value, that is, to set thefrequency of the reference signal corresponding to k=0 to a highervalue. For instance, ωm(kT) is given by the following equation (3):$\begin{matrix}{{\omega_{m}({kT})} = {2{\pi \cdot 4500000 \cdot \left( {1 - {\frac{k}{4095} \cdot \frac{4}{9}}} \right)}}} & (3)\end{matrix}$

In this equation (3), ωm(kT)=4.5 MHz at k=0, and ωm(kT)=2.5 MHz atk=4,095. The following equation (4) or (5) may be employed, too, as anexample for decreasing ωm(kT) in the secondary function of k:$\begin{matrix}{{\omega_{m}({kT})} = {2{\pi \cdot 3500000 \cdot \left( {1 - {\left( \frac{k}{4095} \right)^{2} \cdot \frac{2}{7}}} \right)}}} & (4) \\{{\omega_{m}({kT})} = {2{\pi \cdot 4500000 \cdot \left( {1 - {\left( \frac{k}{4095} \right)^{2} \cdot \frac{4}{9}}} \right)}}} & (5)\end{matrix}$

Next, measures for coping with harmonic imaging in the ultrasonicdiagnosis device of Embodiment I will be described.

Let's consider the signal obtained by discretizing the reception signalof the nth element of the array when the center frequency of thetransmission wave is ωs and its envelop shape is A(t). Then, thecomponent having the largest amplitude can be expressed by the followingequation (6):

fn(kT)=u(kT−τn)=A(kT−τn)[exp{j(τs(kT)kT−φn)}+exp{−j)ωs(kT−φn)}]  (6)

where τn is the propagation time of the sound wave to the nth element,and φn=ωs(t)·τn. The value ωS(kT) represents that the frequency givingthe greatest reception amplitude changes gradually depending on thespectrum shift described above. On the other hand, the reception signalcontains also the harmonics, and when the double harmonics are takeninto particular consideration, the discretized reception signal can beexpressed by the following equation (7):

fn(kT)=u(kT−τn)=A(kT−τn)[exp{j(2ωs(kT)kT−2φn)}+exp{−j(2ωs(kT)kT−2φn)}]  (7)

This harmonic imaging based on the double harmonics can be executed byapplying a reference signal the frequency of which satisfies therelation ωm(kT)≈2ωs(kT) as the reference signal to be supplied to themixing means 103. In other words, the discretized digital referencesignal hn(kT) satisfies the following equation (8):

hn(kT)=exp(j2ωs(kt)kT)  (8)

Then, the multiplication results by the mixing means 103 for theabove-mentioned equations (7) and (6) are given by the followingequations (9) and (10), respectively:

gn(kT)=fn(kT)·hn(kT)=A(kT−τn)[exp{j(4ωs(kT)kT−2φn)}+exp(2φn)]  (9)

gn(kT)=fn(kT)·hn(kT)=A(kT−τn)[exp{j(3ωs(kT)kT−φn)}+exp{j(ωs(kt)kT+φn)}]  (10)

In other words, when the reference signal having the frequencyωm(kT)≈2ωs(kT) is multiplied, the double harmonic component of thereception signal shifts to the band having the center at 4ωs(kT) and thebase band of the low frequency. Therefore, the device construction shownin FIG. 1 can extract only this base band and can execute imaging by thefilter effect due to accumulation. At this time, the basic wavecomponent of the reception signal shifts to the frequencies 3ωs(kT) andωs(kT) as expressed by equation (10) and attenuates due to the filtereffect of accumulation. In other words, in the device constructionexplained with reference to FIGS. 1 and 2, harmonic imaging can beexecuted by only rendering the digitized reference signal that is storedin the mixing data storing means 110 and is to be read out seriallysubject to equation (8).

This harmonic imaging, too, is affected by the high band attenuation ofthe reception signal with the lapse of time after wave transmissiondescribed already. Therefore, image quality can be improved by using incombination the control method that lowers the frequency of thereference signal with the lapse of time. When the attenuation effect bythe accumulating means 104 is not sufficient, an analog filter may beinterposed in the prior stage of the digital converting means 102 sothat the basic wave component near the frequency ωs of the receptionsignal can be damped in advance.

(Embodiment 2)

As described above, the band of the reception signal can be selected andimaged by selecting the frequency of the digitized reference signal tobe multiplied to the ultrasonic reception signal. Based on thisprinciple, therefore, Embodiment 2 of the present invention shown inFIG. 4 transmits a plurality of ultrasonic beams having mutuallydifferent frequencies, distributes the outputs of the mixing means withchange-over of the reference signal and acquires in parallel the imagesof a plurality of beams having different frequencies.

Referring to FIG. 4, a driving source 81 generates a transmission wavesignal having a center frequency 1 and a driving source 82 generates atransmission wave signal having a center frequency ω2. Thesetransmission wave signals are inputted to separate wave transmissionfocusing circuits 86 and 87, respectively, to obtain N transmission wavesignals to which a delay distribution is given. Adding means 88 addsthese signals for each channel and drives each element of a probe 101.Here, the focus position F1 realized by the wave transmission focusingcircuit 86 and the focus position F2 realized by the wave transmissionfocusing means 87 have different directions as shown in FIG. 5. Inconsequence, two ultrasonic beams having mutually different centerfrequencies and directions are simultaneously transmitted. Moreconcretely, ω1 is 3.5 MHz and ω2 is 5.0 MHz.

On the other hand, mixing means 103 multiplies the digitized receptionsignal of each element by the reference signal in exactly the same wayas in the device of Embodiment 1. However, this embodiment is differentfrom Embodiment 1 in that the reference signal used is alternatelyswitched between the even-numbered samples and the odd-numbered samples.In other words, memory means inside mixing data generating means 107stores the following digitized reference signals h(kT) in such a manneras to correspond to the sample number k(0≦k<4096) of the receptionsignal:

When k is even-numbered:

h(kT)=exp(jωml(kT)kT)  (11-1)

When k is odd-numbered:

h(kT)=exp(jωm2(kT)kT)  (11-2)

Here, ωm1 (kT) is substantially equal to the transmission wave centerfrequency ω1 of the driving source 81, and ωm2 (kT) is substantiallyequal to the transmission wave center frequency ω2 of the driving source82. These reference signal frequencies ωm1 (kT) and ωm2 (kT) may be aconstant value, respectively, or may be decreased gradually with theincrease of k in such a manner as to correspond to the spectrum shift ofthe reception signals with the lapse of time from the wave transmission.FIG. 6 shows the relation between k and ωm1 (kT) and between k and ωm2(kT) when the reference signals based on the frequency of whichdecreases gradually are employed.

The multiplication results, that are obtained when the digitizedreference signals switching alternately are supplied serially, areassorted by a multiplexer 112 shown in FIG. 4 to even-numbered resultsand odd-numbered results, and are added by adding means 103-a and 104-bthat are disposed double. The addition results are delayed and added byreceive focusing circuits 105-a and 105-b that are disposed double. Thereceive focusing circuit represents together the digital delaying means105 and the adding means 106 shown in FIG. 1. The wave receptionfocusing circuit 105-a is controlled so that the focus of the receptionwave beam is formed in the direction of the transmission wave focus F1with respect to the odd-numbered samples as the product of themultiplication result of the reference signals having the frequency ωm1(kT). In other words, the delay distribution is imparted so as tocompensate for the difference of the ultrasonic wave transmission timefrom the sound source in the direction F1 to each element. The wavereception focusing circuit 105-b is controlled in such a manner as toform the focus of the reception wave beam in the direction of thetransmission wave focus F2 with respect to the even-numbered samples asthe product of the multiplication result of the reference signals havingthe frequency ωm2 (kT). In other words, the delay distribution isimparted so as to compensate for the difference of the ultrasonic wavetransmission time from the sound source in the direction F2 to eachelement. These reception wave focuses are moved from near to far in thedirections F1 and F2 with the lapse of time from the transmission of thewaves. In other words, it is preferred to conduct reception wave dynamicfocusing in the same way as in Embodiment 1. These beamforming resultsare converted to the absolute values by an envelop converting circuit 92and written as separate scan line data of the tomograms into a scanconverter 94.

FIG. 7 shows the mode of assortment of the reception signal samplesobtained sequentially and beamformed in multiplicity, for ease ofunderstanding. Incidentally, FIG. 4 shows the device constructioncomprising the multiplexer 112 and the double accumulating means 104-aand 104-b, but means in the next stage to the mixing means 103 may beprocessing means that have the functions of alternately assorting themixing results and adding them. Such assorting and multiplex phasing canbe realized because sampling is made beyond the Nyquist frequency of thereception signal in the stage of the digital converting means 102 and asa result the output of the mixing means 103 is obtained in a cycle farsmaller than the required sampling cycle.

The operation described above gives two scan lines in parallel, that isconstructed by two ultrasonic beams having different frequencies, withone wave transmission operation. In consequence, high-speed imaging canbe made. When compared with the conventional system that simultaneouslygenerates only a plurality of reception beams, this embodiment issuperior in the aspect of lateral resolution because the transmissionwave beams, too, have the independent focuses, respectively.

Various modifications can be made by applying the simultaneoustransmission by two frequencies ω1 and ω2 in this Embodiment 2. One ofsuch modifications is the removal of speckles. The term “speckle” meansthe interference pattern of the ultrasonic signals inside the livingtissue. In practical diagnosis, there is an undesirable case in which astrong speckle is observed at portions where reflecting bodies do notactually exist. Since the speckle is the interference pattern, theappearing positions of the speckle vary when the frequency of thetransmission wave signal varies. On the other hand, the signal from theactual reflecting body appears at the same position even when thefrequency of the transmission signal varies. In other words, when thereflecting signals from the same position by the two transmissionsignals having mutually different frequencies are summed, the signalsfrom the actual reflecting body strengthen each other while the specklesignals weaken each other. As a result, the speckles on the image can bedecreased as is known in the art.

It will be assumed hereby that in the ultrasonic diagnosis device shownin FIG. 4, the focus F1 realized by the wave transmission focusingcircuit 86 and the focus F2 realized by the wave transmission focusingcircuit 86 exist at the same position. The mixing means 103 executesmultiplication by using alternately the reference signal obtained fromthe signal having the frequency near ω1 and the reference signalobtained from the signal having the frequency near ω2. The doublereceive focusing means 105-a and 105-b execute the same beamformingprocess so that the phase is in alignment with the ultrasonic wave fromthe direction of the focus at the same position. These two beamformingresults are summed and are then converted to the absolute values by theenvelop converting circuit 92 to give the image data. In consequence,the two ultrasonic beams having the frequencies ω1 and ω2 formedsimultaneously from one reception signal are added, and the speckles canbe reduced in real time. Strictly speaking, however, it is not the dataat the same timing that are added to each other, but the data deviatedfrom each other by one sampling cycle T. Nonetheless, no problemdevelops at all as to the removal of the speckles.

Next, an example of the application of Embodiment 2 to simultaneousmulti transmit focusing will be explained. In this case, the focus F 1and the focus F2 realized by the two wave transmission focusing circuits86 and 87 shown in FIG. 4 have the same direction as shown in FIG. 8.However, the focus F1 takes a near distance with the focus F2 taking afar distance. If the center frequency ω1 of the first beam is 3.5 MHz,for example, the center frequency ω2 of the second transmit beam thatare synthesized and transmitted with the first beam is set to about 2.7MHz, so as to cope with the spectral shift of the far distance echo. Theconstruction and operation of each means subsequent to the mixing means,and the digitized reference signals employed, are exactly the same asthose of Embodiment 1 explained with reference to FIG. 1 to FIG. 3. Thefrequency of the digitized reference signal is decreased gradually withthe lapse of time from wave transmission so as to cope with the spectrumshift of the reflection echo. In the case of the wave transmissionfrequency described above, the frequency of the reference signal isvaried from 3.5 MHz toward 2.5 MHz, for example.

Under such wave transmission and reception signal processing, in theprocessing of a reception signal from a shallow depth, since thefrequency of the reference signal used is approximate to ω1, areflection response strongly reflecting the transmit beam of ω1 thatconverges in the near distance is obtained. On the contrary, in theprocessing of a reception signal from a deeper depth, a reflectionresponse strongly reflecting the transmit beams of ω2 that converges inthe far distance is obtained because the reference signal used isapproximate to ω2. In other words, the resulting ultrasonic tomogram hashigh resolution in the lateral direction over a broad depth range.Moreover, because the ultrasonic beams having different convergingdistances are synthesized and transmitted simultaneously, the scan timedoes not increase and high-speed imaging can be conducted.

The invention completed by the present inventor has thus been explainedconcretely on the basis of the embodiments thereof. However, the presentinvention is not particularly limited to these embodiments but can benaturally modified in various ways without departing from the scopethereof.

INDUSTRIAL APPLICABILITY

The effects brought forth by the typical inventions disclosed in thisapplication are briefly as follows.

(1) The frequency of the reference signal can be freely controlled inthe digital ultrasonic diagnosis device in accordance with the frequencyshift beamforming method, and various high-quality or high-speed imagingcan be made in consequence.

(2) Harmonic imaging can be done.

(3) High-speed imaging can be done as a plurality of beams havingmutually different frequencies are formed simultaneously.

(4) Resolution in the leteral direction can be improved in a broad depthrange, and quality of the ultrasonic images can be improved.

What is claimed is:
 1. An ultrasonic diagnosis device comprising: wavetransmitting means for driving repeatedly an ultrasonic element groupdisposed in an array, and generating ultrasonic waves; digitalconverting means for sampling reception signals obtained by detectingreflection echoes in said ultrasonic element group at a samplingfrequency higher than a Nyquist frequency of an upper limit of a signalband of said reception signals and converting them to digital signals,respectively; mixing means for multiplying said digital signals byreference signal; accumulating means for accumulating the digitalsignals multiplied, a number of said digital signals accumulated beingequal to a number of a plurality of samples; wave reception focusingmeans for imparting a delay for aligning a phase difference peculiar toeach of said ultrasonic elements to said digital signals accumulated,and adding said digital signals; and mixing data generating means forserially supplying data obtained by discretizing a signal having afrequency which is multiple of a center frequency of a transmitted waveand is changed with a lapse of time from a wave transmission, to saidmixing means as said reference signal, wherein said mixing datagenerating means includes data computing means for computing in advancediscrete values of said signal having the frequency which is multiple ofthe center frequency of the transmitted wave and memory means forstoring said discrete values computed, and for serially outputting saiddiscrete values stored under control of read addresses.